1. Field of the Invention
The present invention relates to a coupler used for transmitting high frequency signals through insulating material.
2. Prior Art
For transmitting high frequency signals through insulating materials, such as glass, etc., it is desirable for the high frequency signals to be tansmitted without damaging the insulating materials. For example, when connecting a communication device installed in a car to an antenna mounted outside of the car, it is desirable not to damage the car.
There are two types of known devices which meet such a requirement: a device using a capacitor coupling and a device using loop coils.
The device using the capacitor coupling includes two electrodes with glass interposed in between forming a capacitor composed of the two electrodes and the glass. High frequency signals are transmitted by means of the electrostatic capacity (capacitance) of the capacitor (condenser). However, this device has disadvantages: transmission loss is relatively great and also, the transmitted frequency characteristics are not uniform.
On the other hand, the device using the loop coil is designed to have two loop coils with a piece of glass placed in between so that electromagnetic coupler is effected between those two loop coils. The advantages of this device are that transmission loss is relatively less and frequency characteristics are uniform.
The above-mentioed device using the loop coil, however, has a problem. In order to reduce transmission loss and to make frequency characteristics uniform, the loop coils must be very large in size. Accordingly, for example, when the device is mounted on the window shield of a car, it obscures visibility.